Nonlinear forcing, response and damping of oscillating water columns

This thesis improves the physical understanding of a dynamically responding offshore OscillatingWater Column (OWC) of a fixed and cylindrical type. The study concerns the analytical models of the linear potential flows and the numerical simulations of the fully nonlinear viscous flows of OWCs undergoing different excitation mechanisms and conditions. Of particular interest in this research are the physical origin and the significance of nonlinearities in the forces acting on a water column. The vortex shedding at the lowermost end of an OWC has a marked influence on the forcing caused by wave excitation at the resonant and a higher frequency. On the other hand, the free-surface nonlinearity has a negligible effect on this force. Furthermore, an evaluation of the hydrodynamic and the air forces demonstrates the diminishing role of the vortex shedding as the amplitude of the response (or motion) of a water column increases. The vortex shedding and the nonlinear inertial forcing are responsible for any nonlinear characteristics of the internal water surface elevation within an OWC. The former causes the progressively decreasing response amplitude operator and progressively varying phase difference of the water elevation, while the latter provokes the response asymmetry. The hydrodynamic coefficients crucially depend on the amplitude of the water column motion, the mouth shape, and the dimensions of an OWC. When flow separation occurs, the added-mass coefficient progressively increases with the amplitude. It is consistent with the departures from linear radiation theory when considering the radiated wave elevation near an OWC with a sharp-edged mouth. Formation of the vortex that primarily contributes to the nonlinear damping coefficient most significantly reduces the response at the resonant frequency. This role of vortex formation is more pronounced under a high- than a low-frequency excitation. These insights and the dominance of the flow driven by the water column (in relative comparison to the wave flow) explain the applicability of the hydrodynamic coefficients evaluated from a forced oscillation test to predict a wave-induced response.Open Acces